System for the automatic tracking of a moving target

ABSTRACT

Pulse-modulated radar system with conical scanning and utilizing two MTI filters. Due to the pulse modulation and the Doppler shift of the echo signals, disturbing frequency components can appear in the range of the scan frequency after coherent detection. This disturbance is avoided by utilizing two quadrature receiver channels. The amplitude of the echo signals is determined by the vector sum of real and imaginary Doppler signal components.

United States Patent 1 [111 3,882,500

Sikora May 6, 1975 [54] SYSTEM FOR THE AUTOMATIC 3,781,886 12/1973 Lanket a1 343/7.4 TRACKING OF A MOVING TARGET 3,796,989 3/1974 Ravas et al.343/7.7 X

[75] Inventor: Harald Sikora, Bremen, Germany Primary Examiner Tl HTubbesing [73] Assignee: U.S. Philips Corporation, New Attorney, Agent,or Firm-Frank R. Trifari; George B.

York, NY. Berka [22] Filed: Nov. 9, 1973 v [21] Appl. No.: 414,532 [57]ABSTRACT Pulse-modulated radar system with conical scanning d t'l' t MTlf'lt [30] Foreign Application Priority Data an u I 12mg W0 1 N l l 1972G man 2255373 Due to the pulse modulation and the Doppler shift of e ythe echo signals, disturbing frequency components can 52 US. Cl.343/7.4; 343/7.7 zggzfg g t z gz of the Scan frequency after [51] Int.Cl G0ls 9/02 [58] Field of'Search 343/7.4, 7.7 Thls dlsturbance 1savolded y utlhzmg two quadrature receiver channels. The amplitude of the5 References Cited echo signals is determined by the vector sum of realUNITED STATES PATENTS and imaginary Doppler signal components.

3,772,696 11/1973 Kummer 343/7.4 3 Claims, 2 Drawing Figures RADAR PULSETRANSMITTER l 7 MODULATOR 11 U 9 U l 5 1 ANGULAR MASTER ERROR MTIDETECTOR FILTER ECHO PULSE 5 U 7 RECEIVER s 14 MASTER PHASE DETECTOR A13 PHASE SHIFTER ADDITION i Q We/ DEV'CE 12\ AUXILIARY PHASE d DETECTORAUXILIARY] Us in! FILTER PMmTmmmfi 3,882 500 RADAR PULSE TRANSMITTER 11U 9 1 5 1 ANGULAR MASTER ERROR P112512 ECHO PULSE DETECTOR 6 U7 RECEIVERs 14 MASTER PHASE DETECTOR 13 1 PHASE SHIFTER ADDITION/ JUS F 11 DEVICE12\ AUXILIARY PHASE DETECTOR u; AUXILIARY/ U5 1 MTI FILTER SQUARE Z= /R+I ROOT 18 DEVICE 16 SUMMING DJ MULTIPLIER DEV'CE Fig.2

SYSTEM FOR THE AUTOMATIC TRACKING OF A MOVING TARGET The inventionrelates to a system for the automatic tracking of a moving target.comprising means for radiating pulse-modulated electromagnetic energy ina rotating beam whose axis describes a cone, means for receiving andselecting electromagnetic energy reflected by a moving target, and meansfor deriving tracking signals from the modulation of the amplitude ofthe received and selected electromagnetic energy.

Pulse-modulated radar systems with conical scan for tracking a movingtarget of the kind set forth are known. The conical scan produces amodulation of the amplitude (scan modulation) of the electromagneticenergy reflected by a target (echo signal) with the scan frequency. Themodulation depth is then dependent of the angle difference between theaxis of the scan cone and the direction of the target.

The normally used method of deriving tracking signals, for tracking thetarget in azimuth and elevation, from the echo signal consists in thephase detection of the envelope of the echo signal, the phase referencebeing two quadrature components ofa reference signal of scan frequency(U.S. Pat. No. 3,026,513, column 1).

In radar systems for tracking moving targets filters (MTI filters) areused in the video portion for selecting the echo signals whosefrequency, due to the movement of the target, has been shifted (Dopplershift) with respect to the electromagnetic energy radiated by the radarsystem. The tracking signals are then derived in the described mannerfrom the envelope of the selected echo signals.

In pulse-modulated radar systems the pulse modulation in the frequencyspectrum of the radiated electromagnetic energy causes frequencycomponents at distances amounting to an integer number of times thepulse repetition frequency from the central carrier frequency ortransmission frequency. After Doppler shifting and coherent detection ofthe echo signal, these frequency components may give rise to signalswhich are situated in the range of the scan frequency, with the resultthat the tracking signals are disturbed and the tracking of the movingtarget is hampered.

The invention has for its object to provide a system of the kind setforth in which the said disturbing influencing of the accuracy at whicha target can be tracked is avoided.

The system according to the invention is characterized in that the meansfor receiving and selecting the electromagnetic energy reflected by amoving target comprise two selective receive channels for coherentdetection at a 90 phase difference of the received electromagneticenergy, each of the two selective receive channels being provided withmeans for selecting the received electromagnetic energy whose frequencydiffers due to the Doppler shifting from that of the radiatedelectromagnetic energy, the two selective receive channels beingconnected to means for determining the amplitude of the detectedsignals, the said am litude corresponding to a complex signal Z R +1where R real component and l imaginary components of the Dopplerfrequency.

The invention will be described in detail hereinafter with reference tothe Figures.

FIG. 1 shows the block diagram of a system according to the invention.

FIG. 2 shows the block diagram of a device for determining theamplitude.

The radar system shown in FIG. 1 comprises a transmitter oscillator 1which is coupled to a pulse modulator 2 which is coupled, via a transmitreceive switch 3, to the antenna 4. The antenna 4 has a small beam widthand is provided with means for rotating the beam such that the axis ofthe beam describes a cone.

In the receiving device the aerial 4 is coupled to a receiver 5 viaswitch 3. In the angular error detecting device 6, which may beconstructed as described in U.S. Pat. No. 3,026,513, the trackingsignals for azimuth and elevation are derived from the envelope of theecho signal.

According to the invention, the output of receiver 5 is coupled to twoquadrature receive channels 7 and 8 for coherent detection at a phasedifference of the echo signals. Each of the receive channels 7, 8comprises a master phase detector 9, an auxiliary phase detector l0 andMTl filters 11, 12.

The reference voltage U for phase detector 9 is directly supplied by thetransmitter l, and the phase detector 10 is supplied by the transmitteroscillator 1 via a 90 phase-shifting network 13.

The output of filters l1 and 12 representing, respectively, real andimaginary components of Doppler echo signals of moving targets areapplied to an adding device 14 for determining the amplitude Zcorresponding to R =1 where R real component and l= imaginary componentof the detected Doppler; echo signals. The amplitude Z is independentfrom the time behavior of the Doppler echo signals.

The block diagram of the device 14 is shown in FIG. 2. Each of theinputs l5 and 16 is coupled to the two inputs of a multiplication stagel7, 18. These stages quadrate the input signals. The output signals ofthe stages 17 and 18 are summed in the summing device 19, and the outputsignal thereof is applied to a square root device 20 for forming theroot of the supplied signal.

The signals applied to the inputs l5 and 16 are the quadraturecomponents of the echo signal, with the result that the signal which isformed on the output of device 20 represents the amplitude of the echosignal. This amplitude is still modulated with the scan frequency anddevice 6 derives the tracking signals from this modulation.

The undesired modulation products resulting from mixing Doppler shiftfrequency of the echo signals with echo pulses are no longer present inthe output signal of device 14, with the result that no disturbingfrequency components can occur in the range of the scan frequency on theoutput of device 14, so that adverse influencing of the accuracy atwhich a target can be tracked is avoided,

The foregoing will be mathematically illustrated. For this purpose,first a few signals will be defined.

The signal U of transmitter oscillator l is represented by:

U 0 cos (2 t The scan modulation is represented by the factor:

l+ m cos (0),! (b) The pulse modulation of pulse modulator l is represented by the time function g (t).

The propagation time of the electromagnetic waves to the target isrepresented by T. The Doppler shift frequency is represented by co Forthe sake of simplicity, the effect of the propagation time on the scanmodulation is ignored, which is permissible for the relevant low scanfrequencies.

The following can then be written for the output signal U, of pulsemodulator 1:

1 0, g(t) cos w! The following can be written for the signal U- which isincident on the target 2:

The following can be written for the signal U which reflected by the(moving) target:

Another scan modulation occurs upon reception of the echo signal, withthe result that the following can be written for the output signal U ofreceiver 5:

After coherent detection in the receiver channels 7 and 8, the signals Uand U are produced:

' where U U,,.

The product of the factors g(I2T) and cos w,,(!2T) may give rise tofrequency components in the range of the scan frequency.

However, if the signals U and U are processed in the device 14 in thedescribed manner, the signal U is produced on the output thereof:

ence signal for the receive channels 7 and 8 is derived from thetransmitter oscillator l, the reference signal could alternatively bederived from a separate oscillator which is synchronized with thetransmitter oscillator. Furthermore, the reference signal need not havethe same frequency as the transmitter oscillator. The receiver 5, havingno other function in the described radar system than to couple theswitch 3 to the channels 7 and 8, could effect a conversion of the echosignal to an intermediate frequency, in which case the frequency of thereference signal for the channels 7 and 8 would have to be equal to theintermediate frequency.

What is claimed is:

l. A radar system for tracking a moving target comprising means fortransmitting pulse-modulated electromagnetic energy in a rotating beamwhose axis describes a cone, means for receiving electromagnetic energyreflected by a moving target, a first phase detector connected to thereceiving means and supplied by a first reference signal from thetransmitting means to compare the phase of the reflected signal with thetransmitted signal, first frequency selecting means connected to thefirst phase detector to pass-through the real signals whose frequencydiffers due to Doppler shifting from that of the transmittedelectromagnetic energy, a second phase detector connected to thereceiving means and supplied by a second reference signal which is phaseshifted about with respect to the first reference signal, secondfrequency selecting means connected to the second phase detector topassthrough the imaginary Doppler signal components, vector adding meansconnected to the first and second filter means for adding the real andimaginary Doppler signal components, and tracking error detecting meansconnected to the output of the said adding means.

2. A system as claimed in claim 1, wherein said vector adding meanscomprises a first multiplier stage for squaring the real Doppler signal,a second multiplier stage for squaring the imaginary Doppler signalcomponent, a summing device for adding the outputs from said multiplierstage and a square root device for providing a square root from theoutput of the summing device.

3. A system as claimed in claim 1, wherein said frequency selectingmeans are MTl filters.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORECTION Patent No.3,882,500 Dated May 6 1975 Inventor 1) HARLD SIKORA It iscertifie-iii;that error appears in the above-identified patent and thatsaid Lettefs Patent are hereby corrected as shown below:

On the title page Section [30] change "2255373" to Column 3, line 10,change "cos 0U t" to cos J1 t--.

Signed and Stalecl this fif h Day of August1975 [SEAL] Arrest.-

RUTH C. MASON C. MARSHALL DANN Arlvsling Officer (mnmixsimu'r uflalenlsand Trademarks UNTTEE STATES PATENT OFFICE QERTHEQATE @F @GRRECTEQNPatent No. 8 Dated May 6 1975 Inventor (3) HARLD SIKORA It is certifiaizi that error appears in the above-identified patent and that saidLettei's Patent are hereby corrected as shown below:

On the title page Section [30] change "2255373" to 0 Column 2, line 30,change R I to Column 3, line 10, change "cos w t" to cos IL t--.

a fiigned and fiealcd this fif Day 9? August1975 [SEAL] Arrest:

RUTH C. MASON C. MARSHALL DANN --iII'S iHX l/fiv ('ummixsimn'r uflalenlsand Trmlmnarks

1. A radar system for tracking a moving target comprising means fortransmitting pulse-modulated electromagnetic energy in a rotating beamwhose axis describes a cone, means for receiving electromagnetic energyreflected by a moving target, a first phase detector connected to thereceiving means and supplied by a first reference signal from thetransmitting means to compare the phase of the reflected signal with thetransmitted signal, first frequency selecting means connected to thefirst phase detector to pass-through the real signals whose frequencydiffers due to Doppler shifting from that of the transmittedelectromagnetic energy, a second phase detector connected to thereceiving means and supplied by a second reference signal which is phaseshifted about 90* with respect to the first reference signal, secondfrequency selecting means connected to the second phase detector topass-through the imaginary Doppler signal components, veCtor addingmeans connected to the first and second filter means for adding the realand imaginary Doppler signal components, and tracking error detectingmeans connected to the output of the said adding means.
 2. A system asclaimed in claim 1, wherein said vector adding means comprises a firstmultiplier stage for squaring the real Doppler signal, a secondmultiplier stage for squaring the imaginary Doppler signal component, asumming device for adding the outputs from said multiplier stage and asquare root device for providing a square root from the output of thesumming device.
 3. A system as claimed in claim 1, wherein saidfrequency selecting means are MTI filters.